Vapor compression air conditioning and refrigeration systems use the common refrigeration cycle to produce cooled air. A typical system 100, such as is shown in simplified form in FIG. 1, uses an electric motor to drive a compressor 102. Compressor 102 increases the pressure in a refrigerant loop 101 and pumps the refrigerant, such as R-22 (a.k.a Freon) or R-410A, under pressure to a condenser 103. Variable speed fan 104 blows air over the condenser 103 causing heat to be removed from the refrigerant. The cooled liquid refrigerant is then sent to an evaporator 106 through a thermal expansion valve (TXV) 105.
A TXV is a component in refrigeration and air conditioning systems that controls the amount of refrigerant flow into the evaporator 105 thereby controlling the heating at the outlet of the evaporator. The evaporator 106 allows the compressed cooled refrigerant to evaporate from liquid to gas while absorbing heat in the process. This state change and heat absorption cool the evaporator. Blower 107 then blows air over the chilled evaporator, thereby cooling the air which can then be forced into the desired rooms or refrigeration chambers. The low pressure, gaseous refrigerant is then returned to the compressor where it is repressurized and sent back to the condenser.
The cooling of the air by the evaporator 106 also has the effect of reducing the amount of water vapor that the air can hold. The water vapor in the air condenses thereby dehumidifying the air as well as cooling it.
In prior art systems, such as system 100, the quantity of refrigerant charge, which is the amount of refrigerant in the refrigerant loop 101, is fixed. The charge quantity used is a compromise because the optimum refrigerant charge changes with the operating mode and ambient conditions. It would be useful to provide a vapor compression refrigeration system that could change the refrigerant charge in the system to improve performance and efficiency under different operating modes and environmental conditions.